Navigation systems are utilized for a variety of entities, such as for aircraft, boats, and submarines. Typically, an inertial navigation system is utilized with a plurality of sensors to provide location information of an entity.
A typical inertial navigation system integrates the information gathered from a combination of gyroscopes and accelerometers in order to determine the current state of the system. Gyroscopes measure the angular velocity of the system in the inertial reference frame. By using the original orientation of the system in the inertial reference frame as the initial condition and integrating the angular velocity, the system's current orientation is known at all times. Accelerometers measure the linear acceleration of the system relative to an inertial reference frame.
However, by tracking both the current angular velocity of the system and the current linear acceleration of the system measured relative to the moving system, it is possible to determine the linear acceleration of the system in the inertial reference frame. Performing integration on the inertial accelerations, while using the original velocity as the initial condition, and using the correct kinematic equations yields the inertial position.
All inertial navigation systems suffer from integration drift. Small errors in the measurement of acceleration and angular velocity are integrated into progressively larger errors in velocity, which is compounded into errors in position. This is a problem that is inherent in every open loop control system.
Inertial navigation may also be used to supplement other navigation systems, providing a higher degree of accuracy than is possible with the use of any single navigation system. For example, if, in terrestrial use, the inertially tracked velocity is intermittently updated to zero by stopping, the position will remain precise for a much longer time, a so-called zero velocity update.
Control theory, in general, and filtering, in particular, provide a theoretical framework for combining information from various sensors. One of the most common alternative sensors is a satellite navigation radio such as a Global Positioning System (GPS). By properly combining the information from an Inertial Navigation System (INS) and the GPS system, the errors in position and velocity are stable.
However, there are some environments where it becomes difficult to address integration drift. For example, on a submarine, it is important to remain underwater for a significant amount of time due to military considerations, and it may be undesirable to resurface within a specified time period. Modern navigation systems require GPS for sustained operations. Accordingly, prolonged GPS outages can severely degrade navigation performance. GPS reception under 100 feet of water becomes difficult, if not impossible, because water attenuates the L-band GPS signal. Therefore, submarines are particularly susceptible to GPS outages during protracted undersea operations. In another example, the vehicle may be underground for an extended period of time where the conventional sensors such as GPS, altimeters, beams, and the like may not be used.
What is needed is a method and system to provide an effective navigation system particularly when a body is underground or under water. The present embodiment addresses such a need.